I bought my 2013 MAC PRO with 12 cores and 16 gigs of RAM. It was a big mistake, since I never maximize the cores but I did max out my RAM all the time.
I figured at purchase it would be easier to update the RAM than cores.
I bought 32 gig's of OWC memory (2x-16) in January 2015 and found the memory not quite as fast and didn't seem to work as efficient as the OEM. I just purchase April 2015 a matching pair of 32 gig's of OWC memory (2x-16) total of 64 gigs of RAM and WOW the MAC PRO is working very well. I'm not sure if the computer likes to have matching pairs or all the memory bay's full.
My memory app is telling me less memory is being used and managed efficiently.
My recommendation if you can afford it, buy the 64 gig (4x-16) I believe these configuration works best.